This invention relates to polypropylene based compositions, films, and labels prepared from such films. More particularly, the invention relates to polypropylene based compositions comprising a mixture of a propylene polymer or copolymer and an alkylene alkyl acrylate copolymer, and films and labels prepared therefrom that are ink printable die-cuttable and/or scuff resistant.
It has long been known to manufacture and distribute pressure-sensitive adhesive stock for labels by providing a layer of face or facestock material for the label or sign backed by a layer of pressure-sensitive adhesive which in turn is covered by a release liner or carrier. The liner or carrier protects the adhesive during shipment and storage and allows for efficient handling of an array of individual labels after the labels are die-cut and the matrix (waste) is stripped from the layer of facestock material up to the point where the individual labels are dispensed in sequence on a labeling line. A typical method of die cutting uses a steel die blade. During the time from die-cutting to dispensing, the liner or carrier remains uncut and may be rolled and unrolled for storage, transit and deployment of the array of individual labels carried thereon.
Failure to reliably dispense is typically characterized by the label following the carrier around a peel plate without dispensing or xe2x80x9cstanding-off xe2x80x9d from the carrier for application to the substrate. Such failure to dispense is believed to be associated with excessive release values between the label facestock material and the liner. Dispensibility also is dependent upon the stiffness of the facestock. Failure to dispense may also be characterized by the wrinkling of the label due to lack of label stiffness at the dispensing speed as it is transferred from the carrier to the substrate. Another particular need in many labeling applications is the ability to apply polymeric-film labels at high line speeds, since an increase in line speed has obvious cost saving advantages.
In many label applications, it is desirable that the facestock material be a film of polymeric material which can provide properties lacking in paper, such as clarity, durability, strength, water-resistance, abrasion-resistance, gloss and other properties. Historically, polymeric facestock material of thicknesses greater than about 3 mils (75 microns) have been used in order to assure dispensability in automatic labeling apparatuses. For example, plasticized polyvinyl chloride films about 3.5 to 4.0 mils (87.5 to 100 microns) thick were used in label application because these films exhibited the desired flexibility characteristics. However, the migration of the plasticizers used in PVC films to convert the normally rigid films to flexible films was recognized as a major problem area for these types of films resulting in loss of desirable properties such as adhesion and flexibility, as well as other problems such as ink anchorage failure, color buildup, and shrinkage. Eventually, migration of the plasticizer results in wrinkling, cracking and visual deterioration of the facestock and/or label. Also, it is desirable to reduce the thickness or xe2x80x9cdown-gaugexe2x80x9d the facestock material in order to attain savings in material costs. Such reduction in facestock thickness often has resulted in reduced stiffness and the inability to die-cut and dispense the labels in a reliable commercially acceptable manner using automatic machinery. There also was pressure for environmental reasons to prepare labels from facestock polymers other than polyvinyl chloride.
Polymeric materials suggested in the prior art as useful in preparing labels include biaxially-oriented polypropylene (xe2x80x9cBOPPxe2x80x9d) of thicknesses down to about 2.0 mils (50 microns). These materials provide cost savings as they are relatively inexpensive, and they have sufficient stiffness to dispense well. However, these materials also have relatively high tensile modulus values in both machine-direction (MD) and cross direction (CD) which results in unacceptable conformability characteristics. When biaxially-oriented films are applied to rigid substrates such as glass bottles, the application is not completely successful. The relatively stiff labels have a tendency to bridge surface depressions and the mold seams resulting from bottle-forming processes resulting in an undesirable surface appearance of the applied label simulating trapped air bubbles. This has somewhat impeded the use of pressure-sensitive adhesive labels to replace prior glass bottle labeling techniques such as ceramic ink directly bonded to the bottle surface during glass bottle manufacturing processes as customers find the appearance unattractive. Such ceramic ink techniques are environmentally undesirable due to objectionable ink components which contaminate crushed bottle glass in recycling processes. Attempts to use the relatively stiff oriented polypropylene films on flexible substrates such as plastic bottles also have not been completely successful because the labels do not have the flexibility required to conform to the flexible plastic containers. Oriented polypropylene films are also more difficult to print than PVC or polyethylene films.
Other useful materials are unoriented polyethylene and polypropylene films that are also relatively inexpensive and conformable. However, both of these films are difficult to die-cut and do not dispense well at low calipers. In Europe, an unoriented, relatively thick polyethylene facestock has been used successfully in preparing labels. The facestock is die-cuttable and the labels can be dispensed in high speed automatic dispensing equipment. The normal thickness of this xe2x80x9cstandardxe2x80x9d polyethylene facestock in Europe is about 4.0 mils (100 microns). Attempts to reduce the gauge of the polyethylene facestock to reduce costs has not yet met with significant success because the thinner polyethylene facestock is not readily die-cuttable with the die leaving a mark on the liner, stringers on the cut label, and/or hangers between the labels. A stringer (also called ticker) is a small thread of material between the label and the matrix after die cutting. Thus, the label and matrix are still connected by a small string of material. A stringer occurs when the label is not clean cut, and it can cause the label to be removed with the matrix. A hanger occurs when a segment of the CD matrix breaks during CD matrix stripping. Additionally, the thinner facestock becomes difficult to dispense at higher speeds over a peel plate because of reduced stiffness.
Since labels are intended to carry information, the printability of especially filmic PSA labels is very important. Printability is defined by the sharpness and brightness of the image and by the ink anchorage. The sharpness is closely related to the surface tension of the print surface. The ink anchorage is often tested by a tape test (Finat test: FTM21). In general PVC is printable with a variety of inks intended to be used with PVC. For polyolefin films there is another range of printing inks in most occasions the inks are waterbased (especially in the US) or designed for UV drying (especially in Europe). In general all polyolefin films can be printed with UV inks after on-press corona treatment, PE being better than PP mainly on ink adhesion. For waterbased inks an additional primer or topcoat is needed to get good ink anchorage.
A polypropylene composition is described which comprises a mixture of (A) a propylene polymer or copolymer and (B) an alkylene-alkyl acrylate copolymer. These polypropylene compositions are useful in the preparation of continuous films such as by extrusion, and the films can be oriented by stretching in the machine direction. The films prepared from the polypropylene compositions of this invention exhibit a variety of useful properties including the desired stiffness, flexibility, printability (both waterbased and UV inks), abrasion resistance and/or die-cuttability to be useful as a facestock in preparing labels. Films prepared from the polypropylene composition also can be used as a base film, and the films can be overlaminated or coextruded with a functional film when specific properties are desired. Machine-direction oriented monolayer and multilayer film facestocks for labels, and adhesive containing film labelstock for use in adhesive labels also are described.
The present invention, in one embodiment, relates to the discovery that the films of the polypropylene compositions are useful as the facestocks in label applications, and in particular, in label applications such as on plastic bottles containing personal care products (e.g., shampoo). The clear films prepared from the polypropylene compositions of the invention also are useful for laminating over other polymer films and film facestocks to provide desirable characteristics to the polymer films and facestocks.